Optical fibers used today as telecommunication lightguides have a glassy, cylindrical core, encased within one or more layers of cladding, through which core light pulses are transmitted. Since the various light rays or modes of a pulse follow different paths within the core, as they reflect back and forth along the boundary of the core and cladding, the pulse length elongates during core travel thereby restricting bandwidth. To prevent this from occurring, fibers used for this purpose have been manufactured with their core having an index of refraction profile that varies radially from the core axis to the core periphery. Ideally, the distribution of refractive indices within the core should be such as to cause all light rays of a pulse to travel along the fiber at the same axial velocity regardless of traversed path length variations. In actuality some deviation from optimum refractive indes distribution of the core occurs during fiber manufacture. The manufacturer must, therefore, monitor this distribution to insure that such variations remain within specified limits.
Several methods have been developed for analyzing the index of refraction profiles of lightguides. One of the earlier, but perhaps most accurate technique, was that known as the slab method. This involves an elaborate, tedious and time consuming preparation of a fiber sample whereby a thin slice is cut from the fiber and polished to a high degree of flatness and parallelism of opposed surfaces. The samples, which are then examined with an interference microscope, act as space objects that displace in the core region the normally straight parallel fringe lines of the microscope output field. The fringe displacements or shifts are proportional to the differences in the indices of refraction within the various radial regions of the core and that of the cladding.
Non-destructive approaches have since been taken in determining profiles. Some of these are disclosed in an article by Hunter and Schreider titled "Mach-Zehnder Interferometer Data Reduction Method for Refractively Inhomogenous Test Objects", Applied Optics, Vol. 14, No. 3 (March 1975), in the article by Marhic, Ho and Epstein titled "Non-Destructive Refractive-Index Profile Measurements of Clad Optical Fibers", Applied Physics Letters, Vol. 26, (1975), and in the article by Kokubun and Iga titled "Precise Measurement of the Refractive Index Profile of Optical Fibers by a Non-Destructive Interference Method", Transactions of the IECE of Japan, Vol. E60, No. 12 (December 1977). The just-described methods, which use transverse lumination in forming interferograms, have had limited accuracy and have only been applicable to fibers having a known class of profile, for example, a parabolic profile. Recovery of the index profiles from the interferograms has also been complex. Accuracy of these methods also decrease as the number of modes increases.
As a result of the just described limitations, a simpler and more rapid method of measuring the index of refraction profile of lightguides has been developed which is known as the near-field scanning technique and which is described in the article by Sladen, Payne and Adams appearing in Applied Physics Letters, Vol. 28, No. 5, page 255 (March 1976). With this technique a short length of fiber is illuminated and the profile determined by observation of the light intensity variation across the fiber output face. This method however has limited accuracy due to the presence of leaky modes, i.e., rays that have been partially reflected from and partially refracted into the cladding, as they travel through the fiber, whose contribution cannot be accurately calculated. To overcome this limitation still another method has been devised which is known as the refracted near-field technique described in the article titled "A New Technique for Measuring the Refractive Index Profiles of Graded Optical Fibers" by W. J. Stewart that appeared in the Proceedings of the Conference on Integrated Optics and Optical Communication, Japan (1977). This technique is relatively straight forward and directly yields the refractive index profile across the entire fiber, including its cladding. The fiber dimensions, core centrality, ellipticity and numerical aperture can be determined. Good resolution is maintained throughout and both single mode and multimode fibers can be analyzed.
As opposed to the original near-field scanning method of measuring profiles the more recently developed, refracted near-field method gains its advantage by using light not trapped by the fiber core which is refracted rather than reflected. With this method a lens, having a numerical aperture substantially larger than that of the fiber, focuses a beam of light on a flat endface of a fiber and scans the focused spot across a fiber diameter. An end portion of the fiber is cleaned so that light may escape to ambience. Part of the light is guided down the fiber while the rest, refracted through an end portion of the fiber, radiates as a hollow cone outside of the fiber. The inner part of this hollow cone does still contain leaky modes, i.e. rays of light that have been partially refracted and partially reflected upon striking the fiber cladding, whose contribution to the total power radiated in the cone of light is difficult to assess. But with this newer technique the leaky modes may be excluded by placing a shield or disc in the cone to prevent the leaky modes, as well as the purely reflected modes in this inner region of the cone in which the leaky modes radiate, from reaching the photodetector situated beyond the disc. A more thorough explanation of this technique may be had by reference to the article titled "Refractive Index Profile Measurement of Optical Fibers by the Refracted Near Field Technique" by K. I. White which was published in the March 1979 issue of Optical and Quantum Electronics. A manner in which alpha (.alpha.), the exponential value of the refractive index x-y profile function, may be determined from the light incident upon the photodetector may also be had by reference to this article.
The refractive near-field technique, however, is still not free of certain practical problems. For example, to align the end of the fiber with the scanning beam it is helpful to illuminate the fiber end with light injection into the other end of the fiber. To do this however the fiber must have substantial length and be routed through the just described disc or shield to a source of illumination. With long fibers however the cladding does not transmit light from one end to the other so that it isn't illuminated. The need for the disc itself and its precise alignment is also a handicap. Furthermore, by injecting light into the other end of a long fiber the cladding is not illuminated which renders it difficult to examine the quality of the fiber break, i.e. its degree of flatness. If a new method and apparatus could be devised that would eliminate the need for the disc, would simplify the lens system and fiber mounting, and be one in which short sample fibers could be used with the both cladding and core illuminated, a distinct advance in the art could be realized. It is the provision of such a method and apparatus to which the present invention is primarily directed.